82 PEACTICAL PHYSIOLOGY 



One Daniell battery is connected by two wires with a Pohl's reverser 

 whereby the direction of the current can be changed ; from the 

 reverser the wires pass by means of a Du Bois key to a pair of 

 unpolarisable electrodes. This is the polarising circuit. The stimu- 

 lating circuit is set up separately for the production of single induction- 

 shocks (Fig. 78). A preparation of the sciatic nerve and gastrocnemius 

 muscle is carefully made from a recently pithed frog, and is placed in 

 a moist chamber ; a pin is fixed through the lower extremity of the 

 femur, and the tendo Achillis is connected by a thread with a lever. 



FIG. 78. Diagram of the experiment on the effects of a constant electrical current 

 upon the excitability and conductivity of nerve. 



The sciatic nerve is placed across the kaolin plugs of the unpolarisable 

 electrodes. The drum can be moved by hand. A minimal stimulus 

 for the nerve is obtained, care being taken to use only the break or 

 make-shock. The minimal contraction is recorded on the stationary 

 drum. 



The current from the polarising circuit is closed in an ascending 

 direction, so that the current enters the nerve on the side near the 

 muscle and immediately above the stimulating electrodes, which are 

 connected with the inductorium. The nerve around the point of entry 

 or anode of the polarising current is depressed in its excitability, and 

 the application of a minimal, or even stronger, stimulus is no longer 

 effective (Fig. 79). The polarising current is short-circuited by the 

 Du Bois key, and by means of the reverser is changed in its direction, 

 so that on opening the Du Bois key the current is descending, and the 

 area of nerve near the stimulating electrodes passes into a condition of 



